1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device and, more particularly, to an active matrix liquid crystal display device in which the residual image phenomenon is prevented.
2. Description of the Prior Art
A liquid crystal display device is adopted in many fields such as a viewfinder for a video camera, a pocket TV, a high-resolution projection TV, a personal computer, and the like. In particular, an active matrix liquid crystal display device using a thin-film-transistor (TFT) as a switching element has a major feature that it can maintain a high contrast even when it performs high-capacity display, and accordingly it has been developed and commercialized actively.
The above active matrix liquid crystal display device widely employs the TN (Twisted-Nematic)-method NW (Normally-White) mode as the liquid crystal display mode. According to the TN method, a panel, which is formed of electrode substrates sandwiching a liquid crystal layer such that liquid crystal molecules are twisted by about 90 degrees, is sandwiched between two polarizing plates. According to the NW mode, two polarizing plates are arranged such that their polarizing axes are orthogonal and become parallel or perpendicular to the major axis of liquid crystal molecules in contact with one substrate.
In this case, when no voltage is applied or a voltage equal to a threshold or less is applied, the liquid crystal display device displays white. When a voltage higher than the threshold is applied, the light transmittance of the liquid crystal display device gradually decreases, and the liquid crystal display device displays black. These display characteristics are obtained because when a voltage is applied to the liquid crystal panel, the liquid crystal molecules are modulated to be aligned in the direction of electric field while untwisting their twist structure.
Even when the molecule orientation is the same, the state of polarization of the transmitted light changes depending on the direction of incidence of light on the liquid crystal panel, so the light transmittance differs in accordance with the direction of incidence. In other words, the liquid crystal panel has view angle dependency. The view angle dependent characteristics of the TN method pose a serious problem, particularly in the image characteristics, in a large-screen liquid crystal display which has been developed widely recent years.
As a means for solving this problem, Japanese Examined Patent Publication No. 63-21907, Japanese Unexamined Patent Publication No. 7-36058, and the like propose so-called transverse electric field method IPS (In-Plane-Switching), with which an electric field is not applied in a direction perpendicular to the substrates, as in the TN liquid crystal display method, but the electric field to be applied to the liquid crystal is set substantially parallel to the substrates, and the direction of liquid crystal molecules is controlled within the substrate surfaces, thus modulating light.
With the IPS-mode liquid crystal display device, even when the user shifts his or her viewpoint, he basically sees only the directions of minor axes of the liquid crystal molecules. Hence, this liquid crystal display device is free from the view angle dependency of the “standing direction” of the liquid crystal, and a wider view angle than in a TN-mode liquid crystal display device and the like can be achieved.
A normally-black IPS liquid crystal display device (shown in FIG. 1) utilizing the TFT characteristics will be described as an example. Normally-black is a display method for a liquid crystal display device, with which the polarizing axes of the polarizing layers are arranged such that the display device displays black when no voltage difference is produced between a pixel electrode 24 for driving the liquid crystal and a common electrode 14 and the liquid crystal is orientated at the initial alignment angle, and such that the display device displays white when a voltage difference is applied between the pixel electrode 24 and common electrode 14 and the liquid crystal is rotated (ideally through 45 degrees) from initial alignment.
The liquid crystal is initially aligned with an inclination of about 15 degrees, as indicated by an alternate long and short dashed line, with respect to the pixel electrode 24 and common electrode 14 which form comb electrodes fitted with each other, and rotates in only a specific direction upon application of a voltage between the pixel electrode 24 and common electrode 14. The absorbing axes of the polarizing plates are aligned with the initial alignment direction of the liquid crystal, and an appropriate retardation Δnd (product of a refractive index anisotropy Δn of the liquid crystal and the effective thickness d0 of the liquid crystal layer) is set, so the display device can perform colorless white display and black display.
Display nonuniformity called residual image often occurs in an active matrix liquid crystal display device, in which when the display device displays characters or figures, even after they are deleted, their images stay to remain on the screen for some time. In particular, in the IPS mode, residual image tends to occur very often when compared to a display method in which the direction of the electric field to be applied to the liquid crystal is set substantially perpendicular to the substrate interfaces.
As a method of solving this residual image issue, for example, according to Japanese Unexamined Patent Publication No. 7-159786, if certain conditions for the physical properties of the liquid crystal, alignment film, and insulating film are met, a time required until a display part and non-display part can be discerned after displaying the same pattern for 30 min and erasing the displayed pattern can be set to 5 min or less.
However, according to the invention described in Japanese Unexamined Patent Publication No. 7-159786, when the resistivity of the liquid crystal is sufficiently decreased, although the residual image phenomenon is decreased, if a fixed pattern is displayed for a long period of time, a residual image sometimes occurs.
Conventionally, various factors were attributed to the residual image which occurs in the IPS mode. For example, due to the necessary arrangement, electrode interconnections are formed on only the active substrate unit, and not on the opposite color filter unit. Therefore, an electric field may enter a light-shielding layer for shielding the color layer in the color filter, the electrode interconnections on the active substrate, and TFT elements, thus causing a residual image.
The operation of the TFT will be described. A TFT element serves as a switch which is turned on/off by the voltage applied to the gate electrode. When a sufficiently negative potential (approximately about −10 V although it may differ depending on the arrangement of the TFT element) is applied to the gate electrode, movement of the charges in a-Si decreases, so a signal voltage from the drain electrode is not transmitted to the source electrode. Thus, the signal voltage is not applied to a pixel electrode electrically connected to the source electrode, either.
When a sufficiently positive potential (approximately about +20 V although it may differ depending on the arrangement of the TFT element) is applied to the gate electrode, movement of the charges in a-Si increases, so a signal voltage from the drain electrode is transmitted through the source electrode and applied to the pixel electrode. When a monochromatic fixed pattern is displayed on the IPS liquid crystal display device, the potential difference between the drain and source electrodes of a TFT element in a pixel that displays white differs from the potential difference between the drain and source electrodes of a TFT element in a pixel that displays black.
Why residual image occurs will be described in more detail. For example, if the TFT element has the characteristics as shown in FIG. 2, a residual image that a part that has displayed white becomes brighter is observed. The ON current of the TFT that has displayed white is almost equal to that of a part that has displayed black. Since only the OFF current decreases, the voltage written in the liquid crystal is discharged by an OFF current lower than that at the part that has displayed white. Therefore, the part that has displayed white becomes undesirably brighter than the part that has displayed black, because the effectively large applied voltage remains in the liquid crystal.
A phenomenon opposite to this also happens. When the ON current for the part that has displayed white decreases as shown in FIG. 3, a phenomenon that the part that has displayed white is displayed darker occurs.
In the conventional IPS liquid crystal display device, a light-shielding layer is formed on a color filter unit side. Light entering a region (non-open portion) other than the pixel electrodes and common electrode which form comb teeth is shielded by the light-shielding layer. In the non-open region, the state of alignment of the liquid crystal is the same as that of the open region, so display is not adversely affected.
For this reason, when white is displayed, the liquid crystal on the TFT element of this pixel is rotated from the initial alignment angle. On the other hand, a liquid crystal on the TFT element of a pixel that has displayed black is not much rotated since the electric field between the drain electrode and source electrode is small.
When fixed monochromatic pattern is continuously displayed for a long period of time, the state of the liquid crystal on the TFT element changes, and is fixed. It takes a certain period of time until the changed TFT characteristics are restored. Therefore, the liquid crystal alignment on the TFT element of the pixel that has displayed black differs from that on the TFT element of the pixel that has displayed white, and the electric field enters amorphous silicon differently. This causes a difference in the TFT characteristics. Then, for example, when half-tone solid display is performed, a pattern which is the same as the previously displayed pattern is visually recognized, that is, the so-called residual image phenomenon occurs.
The present inventors found that residual image which was not conventionally solved in the liquid crystal display device was related to a change in TFT characteristics of each pixel. More specifically, according to the findings of the present inventors, when the liquid crystal display device displays a fixed pattern, a change occurs in the characteristics of the TFT element of a pixel which displays white and of the TFT element of a pixel which displays black, thus causing a residual image.
The alignment state of the liquid crystal molecules on the TFT element gradually shifts from the initial state due to the electric field generated between the drain electrode and source electrode. Due to this shift, the electric field enters the amorphous silicon portion of the TFT element differently in the case of white display and in the case of black display. As a result, the subsequent display state changes.